We propose to fill a crucial gap in our knowledge of how influenza is transmitted by characterizing the virus aerosols generated from the respiratory tract of persons naturally infected with human influenza and concurrently determine the ability of readily available personal protective equipment (masks) to mitigate transmission of these infectious particles at the source of generation. Pandemic influenza in 1918, 1957, and 1968 produced widespread morbidity and mortality; in 1918 extensive infection resulted in disruption within communities and breakdown of essential services. Current strategies for limiting a pandemic include use of vaccines, antiviral medications, and because of the lag time for vaccine production, limited supply of antivirals, and explosive pandemic transmission, widespread application of social distancing is also anticipated. Designing effective social distancing interventions requires detailed knowledge of the mechanisms of influenza transmission. Epidemiologic and experimental evidence suggests that influenza is transmitted to a large degree via the airborne route. However, no definitive information is available on the size distribution or rate of infectious particle release by influenza patients, and no studies have examined how to control release or dissemination of influenza aerosols. This project addresses community mitigation and protective measures directed at the spread of influenza virus. The results of this study will have implications on simple and low tech interventions applicable in both the United States and countries in other stages of development. This project is organized around tests of the following hypotheses: (1) Exhaled breath of influenza patients contains airborne viral particles and the size and number of infectious particles is a function of respiratory maneuver (tidal breathing, talking, coughing). (2) Infectious particles have relatively large aerodynamic diameters as they exit the nose and mouth, and surgical masks worn by infected patients can capture a significant portion them. The information gathered by this project will provide a firm scientific basis for effective public health interventions that will be necessary during potential upcoming influenza pandemics to protect communities and prevent widespread economic and social disruption while limiting the spread of infection.